The present invention relates generally to processes for preparing aminobenzoate esters. More particularly, the present invention relates generally to a process for the preparation of an aminobenzoate ester of an alcoholic organic compound containing one or more hydroxyl groups. In certain embodiments, the alcoholic organic compound has an oligomeric or polymeric backbone having from 2 to 300 repeating units.
Aminobenzoate esters are useful as oligomeric amine curatives in a wide variety of applications such as, but not limited to, coatings, adhesives, castable elastomers, molded products, and tougheners or flexibilizers for epoxy-based materials. The reaction to produce these organic esters is typically a transesterification reaction such as that depicted in Equation 1 wherein an alkyl aminobenzoate (1) reacts with an alcohol reagent (2) in the presence of a catalyst (not shown) to form the aminobenzoate ester (3) and the corresponding byproduct alkyl alcohol (4). This reaction is typically an equilibrium reaction that proceeds nearly to completion if an excess of one reactant is used and the product is removed from the reaction mixture such as by, for example, distillation or fractionation.

One alkyl aminobenzoate that can be used to produce the aminobenzoate ester is ethyl 4-aminobenzoate, also known as benzocaine. However, the relatively high vapor pressure of ethyl 4-aminobenzoate may cause it to be lost from the reaction mixture thereby resulting in a reduction in yield of aminobenzoate ester end-product as well as the deposition of solid ethyl 4-aminobenzoate in the reaction equipment. As a result, the typical industry practice is to use a less volatile reagent such as butyl 4-aminobenzoate, also known as butamben, which can be added to the reaction mixture separately or formed in the reaction mixture in situ. The use of butyl 4-aminobenzoate to produce the aminobenzoate ester has some drawbacks. At the present time, butyl 4-aminobenzoate is a less economical reagent than ethyl 4-aminobenzoate. Further, the reaction of butyl 4-aminobenzoate with the alcohol reagent is considerably slower than that of ethyl 4-aminobenzoate.
Yet another solution to prevent the loss of ethyl 4-aminobenzoate from the reaction mixture may be to introduce a hydrocarbon solvent such as, for example, toluene or xylene, into the reaction mixture. The hydrocarbon solvent provides a reflux thereby keeping the reagents within the reaction mixture and improving the reaction efficiency. However, this solution generally requires the additional step of removing these solvents from the end-product such as, for example, by stripping. Further, since many of these solvents present health hazards and because their chemical properties are quite different from the reagents, by-products, and products present in the reaction, it is generally necessary to remove them to very low levels to produce a commercially viable product.
Although preparation of aminobenzoate esters and other organic esters are known in the art, improved methods of making aminobenzoate esters are still sought by those skilled in the art. For example, U.S. Pat. No. 6,111,129 or the '129 patent teaches the production of alkanediol-diaminobenzoates which are useful as curing agents for the production of polyurethane ureas. The alkanediol-diamenzoates are prepared by a transesterification reaction of an alkyl-p-aminobenzoate with an unpolymerized diol—typically in a stoichiometric ratio of at least 2:1 or greater alkyl-p-aminobenzoate to diol—in the presence of a transesterification catalyst. The '129 patent discloses benzocaine as an alkyl-p-aminobenzoate that can be used in the reaction. The '129 patent further teaches the subsequent purification of the alkanediol-diaminobenzoate product by crystallization using an alcohol.
Similarly, U.S. Pat. No. 4,732,959 or the '959 patent teaches the preparation of poly(urethane)ureamides by reaction of a polyisocyanate and a p-aminobenzoate derivative of a polyester polyol. This latter derivative is prepared by transesterifying a polyesterpolyol with an aminobenzoic acid alkyl ester. Example 1 of the '959 patent teaches the transesterification reaction of a lactone-type polyesterpolyol with ethyl p-aminobenzoate or benzocaine and the removal of the byproduct of the reaction, or ethanol, as it was formed. Example 1 also teaches removing unreacted benzocaine from the reaction mixture by heating the mixture to 200° C. under vacuum for 3 hours to provide the polyesterpolyol derivative.
U.S. Pat. No. 5,792,800 or the '800 patent discusses the use of aminobenzoate esters as rubber-to-wire adhesion promoters. Example 1 of the '800 patent describes preparing octadecyl-4-aminobenzoate through the transesterification reaction of benzocaine with 1-octadecanol in a 1:1 stoichiometric ratio in a reaction mixture containing mixed xylenes.
The reference “Development of the Method of Novacain Production” by M. G. Abdullaev, Pharmaceutical Chemistry Journal, Vol. 35 No. 10 (2001), pp. 556-559 or “Abdullaev” teaches the synthesis of novocain using ethyl p-nitrobenzoate as a starting reagent by performing the hydrogenation and transesterification reactions simultaneously using a catalyst system that is active for both reactions. The simultaneous reactions are conducted at a relatively low temperature or around 45° C. Because of this relatively low temperature, the volatility of ethyl 4-aminobenzoate within the reaction mixture and deposition of solid ethyl 4-aminobenzoate in the reaction equipment may not be a concern.
Accordingly, there is a need for process for making aminobenzoate esters of an alcoholic organic compound using alkyl aminobenzoates without yield losses, slow reaction time, and/or operational problems associated with the volatility of the alkyl aminobenzoates.